1. Field of the Invention
The present invention relates to an image processing apparatus, image forming apparatus, and control method thereof and, more particularly, to an image processing apparatus, image forming apparatus, and control method thereof, which form an image using the number of tones smaller than that of an input image.
2. Description of the Related Art
As image output apparatuses for wordprocessors, personal computers, facsimile apparatuses, and the like, printing apparatuses which print information of desired text, images, and the like on sheet-like printing media such as paper sheets, films, and the like are used. Such printing apparatuses adopt various printing schemes. Of these schemes, schemes for forming an image on a printing medium by attachment of printing agents are popularly put into practical use. As a representative example of such schemes, an ink-jet printing scheme is known.
A printing apparatus, which adopts the ink-jet printing scheme, comprises a nozzle group prepared by integrally arraying a plurality of ink ejection ports (nozzles) that can eject ink with an identical color and density, so as to attain improvement of the printing speed and higher image quality. Furthermore, in order to attain higher image quality, some printing apparatuses comprise a nozzle group which can eject ink with an identical color but different densities, and a nozzle group which can eject ink with an identical color and density by changing an ejection amount in some levels.
In such printing apparatus, as a scheme for converting multi-valued input image data into a binary (or N-ary: N is an integer equal to or larger than 2, which corresponds to the number of tones smaller than that of the input image data) image as a dot printing signal, an error diffusion method is known. According to the error diffusion method, pseudo tone expression is implemented by diffusing a binary error (or an N-ary error) produced at a given pixel to a plurality of subsequent pixels.
In addition to the aforementioned error diffusion method, as a means for converting multi-valued input image data as a binary (or N-ary) image as a dot recording signal, a dither matrix method is known. According to the dither matrix method, pseudo tone expression is implemented by executing N-ary processing by comparing a threshold matrix prepared in advance with the multi-valued input data. Since the dither matrix method requires processing simpler than the error diffusion method, it can achieve higher-speed processing.
In order to improve image quality upon actually forming an image, the number of tones of which has been converted by the error diffusion method or dither matrix method, on a printing medium, a technique for determining the formation order and allocations has been proposed (for example, see Japanese Patent Laid-Open No. 2000-103088 (patent reference 1)). According to this technique, by applying the error diffusion method for each scan, even when registrations of respective scans have varied, an image quality drop due to density nonuniformity can be suppressed. More specifically, a plurality of main scans are made on an identical main scan printing region using different nozzle groups, and a binary (or N-ary) image is formed by the error diffusion method for respective main scans. When a binary (N-ary) images is generated by executing the error diffusion method for respective main scans, the dispersiveness of dot allocations in the main scan becomes high and uniform. Therefore, upon forming an image by a plurality of main scans, even when physical registrations such as the feed amounts of a printing medium, the positions of printing elements, and the like have varied, the graininess hardly changes. Since the dot allocations among the plurality of main scans have low correlation, even when the registrations have varied, a change in dot coverage with respect to the sheet surface is reduced, thus greatly relaxing density nonuniformity.
However, with the scheme described in aforementioned patent reference 1, since the dot allocations among the main scans have low correlation, low frequency components of an image generated by the plurality of scans are unwantedly emphasized. These low frequency components are emphasized more with increasing number of scans, and are recognized as graininess which is visually obtrusive.
The present invention has been made to solve such problems, and has as its object to provide an image forming apparatus having the following functions and a control method thereof. More specifically, by executing N-ary processing (N is an integer equal to or larger than 2) based on predetermined constraining condition information for each main scan, deterioration of graininess depending on low frequency components of an image can be suppressed, and a high-quality image can be formed.